This invention relates to apparatus for diminishing tightening torque variations in a screw or nut runner during use in tightening screw joints.
It is a well recognized phenomenon that when using a nut runner for tightening screw joints the final tightening torque obtained is not the same at hard and soft joints despite a specific maximum torque level being set on the tool. At soft joints the obtained final torque is substantially equal to the torque level predetermined by the tool setting, but at hard joints there is always obtained too high a torque.
The reason why an increased torque is obtained at hard joints is that when a hard screw joint is finally tightened the torque resistance arising therein increases very fast or instantaneously.
Though a nut runner is constructed so as to interrupt its power delivery as soon as the torque has reached its pre-set torque level, there is still generated a higher torque. The reason is that the desired torque level is reached very abruptly so that the rotating parts of the nut runner still have an inertia factor related to kinetic energy. This remaining kinetic energy is transferred to the joint in the form of an additional dynamic torque component.
Such a torque addition does not occur at soft joints because the kinetic energy of the rotating nut runner parts is successively absorbed by the joint in advance of reaching the desired torque level so at the end of the tightening operation there is no kinetic energy left to be transferred to the joint.